The information coding part of DNA, the letters of the genetic code. The sequence of bases on a stretch of DNA (i.e., the sequence of As, Ts, Gs, and Cs) determines what the DNA does  if it codes for a protein, turns on a gene, or whatever. In protein-coding regions, three bases code for a single amino acid. For example, the base sequence ATG codes for the amino acid methionine. In a strand of DNA, bases are paired and are lined up across from one another: A pairs with T and G pairs with C.